I play with two regular partners, one when I'm doubles in league play and another playing w/ a regular group - all are 4.5s.
League, because he is extremely quick, we use: Open - go, Closed - stay and one more. We will mix in I formation. I'm right handed. So if we are in I formation and I'm strattling the center line or really close to it, w/my left hand behind my back, I show my pinky finger down if I am going to my left and show my index finger down if I'm going to my right. This works for duece and ad service because I am only showing hte direction I am headed.
In my regular group, my partner has some booming ground strokes but quickness is not his strength. So, we do two things. I use the same open/closed hand signals and we poach when receiving serve. So, if he is receiving I'll hold one finger down or two - meaning I'll poach off of his ground stroke after the return or potential next ground stroke.
Obviously, all of this requires good communication - knowing your partner is comfortable w/differnent looks and positions.
Finally, all of the tactics vary depending on the strengths/weaknesses of the opponents you are playing.